Studies on the hot corrosion of a nickel-base superalloy,Udimet 700

The hot corrosion of a nickel-base superalloy, Udimet 700, has been studied in the temperature range 900–950°C. The effect of the amount of Na ₂SO₄ on the corrosion kinetics was determined. Large weight gains and severe corrosion were associated with two different modes of degradation: (1) formation of large, interconnected sulfides beneath the external scale, and (2) formation of a Na₂MoO₄-MoO₃ melt. The corrosion due to formation of the Na₂MoO₄-MoO₃ melt occurred for all the salt-coating thicknesses, whereas, large sulfides were formed only for the heavier coatings of Na₂SO₄. The formation of Na₂MoO₄-MoO₃ melt required an induction period, and the length of the induction period was observed to be a function of the amount of Na₂SO₄ and of temperature.Work funded under NASA Grant NCC 3-43.     

Sulfate-induced hot corrosion of nickel

Nickel specimens with layers of Na₂SO₄ deposited on the metal surface have been reacted in O₂+4% SO₂ in the temperature range 660–900°C. At temperatures from 671°C (the eutectic temperature of Na₂SO₄+NiSO₄ liquid solutions) to 884°C (the melting point of Na₂SO₄), molten Na₂SO₄+NiSO₄ is formed in the scales above critical pressures of SO₃, and the molten sulfate causes accelerated hot corrosion of nickel. The rapid hot corrosion is preceded by an incubation period during which Na₂SO₄+NiSO₄ solid solutions and eventually molten sulfate are formed. The critical SO₃ pressures for formation of molten sulfate as a function of temperature have been delineated through experimental observations, and these are in agreement with theoretical estimates. When only solid solutions of Na₂SO₄+NiSO₄ can be formed, the reactions are slower than specimens with no Na₂SO₄ layer. The reaction mechanism is concluded to involve inward transport of SO₃/NiSO₄ and of oxygen through the molten sulfate distributed as a network in the NiO layer of the outer part of the scale. Beneath the NiO/molten sulfate layer, the scale consists of NiO with a network of Ni₃S₂. Sulfur, present as (Ni-S)_liq, is enriched at the metal/scale interface. Nickel diffuses outward through the Ni₃S₂ network in the inner layer to the boundary of the NiO/molten sulfate layer, where it reacts with the inwardly diffusing oxygen and SO₃/NiSO₄. The enrichment of sulfur next to the metal is concluded to be due to inward sulfur transport in the NiO+Ni-sulfide layer.     

ESCA study of oxidation and hot corrosion of nickel-base superalloys

A study of the high-temperature oxidation and Na₂SO₄-induced hot corrosion of some nickel-base superalloys has been accomplished by using ESCA to determine the surface composition of the oxidized or corroded samples. Oxidation was carried out at 900 or 1000°C in slowly flowing O₂ for samples of B-1900, NASA-TRW VIA, 713C, and IN-738. Oxidation times ranged from 0.5 to 100 hr. Hot corrosion of B-1900 was induced by applying a coating of Na₂SO₂ to preoxidized samples, then heating to 900° C in slowly flowing O₂. Corrosion times ranged from 5 min to 29 hr. For oxidized samples, the predominant type of scale formed by each superalloy was readily determined, and a marked surface enrichment of Ti was found in each case. For corroded samples, the transfer of significant amounts of material from the oxide layer to the surface of the salt layer was observed to occur long before the onset of rapidly accelerating weight gain. Some marked changes in surface composition were observed to coincide with the beginning of accelerating corrosion, the most striking of which were a tenfold decrease in the sulfur to sodium ratio and an increase in the Cr(VI) to Cr(III) ratio. Supported by NASA Grant No. NSG-3009     

Effects of SO2 and SO3 on the Na2SO4 induced corrosion of nickel

The effects of SO₂ and SO₃ in the environment on the hot corrosion behavior of Ni in the temperature range 750–950°C has been studied. Below the melting point of Na₂SO₄ (884°C), rapid corrosion takes place by formation of a Na₂SO₄-NiSO₄ melt, which can penetrate the porous oxide scale and give rise to sulfide information by coming in contact with the metal. The distribution of the sulfides depends on the SO₂ level in the ambient gas. Continued corrosion occurs by a sulfidation-oxidation mechanism. At temperatures above the melting point of Na₂SO₄, accelerated degradation occurs via dissolution of the surface scale, followed by reprecipitation of the oxide in a nonprotective form.Deceased     

Pitting corrosion of sheets of a nickel‐base superalloy

Cold rolled sheets of a nickel‐base superalloy are used in building gas turbine engines. After rolling to the final gauge, the sheets were annealed at 1180 °C. Pickling of the sheets was carried out to remove the scale and the oxidized layer. It was observed that there was heavy incidence of pitting with perforations at several locations due to pits penetrating through the thickness. The sheets got rejected, resulting in heavy loss. The superalloy contains 25 wt% chromium, 15 wt% tungsten. Tungsten provides high resistance to pitting corrosion. With such a high level of W, the grade has a high Pitting Resistance Equivalent. Further it is a single‐phase material. Pitting attack is thus totally unexpected. The failure was analyzed. Pickling process for this material normally involves the usage of salt bath, sulfuric acid, nitric acid, and/or hydrofluoric‐nitric acid baths. In the case under study, satisfactory pickling was not achieved this way; discoloration and deposits were persistently present on the sheet surface. In order to improve the surface quality, the sheets were dipped in hydrochloric acid bath for a long time. Hydrochloric acid bath was in aged condition; by that point of time, the bath went through extensive usage for pickling of different grades of steels. Consequently there was significant accumulation of iron salts in the bath. It is known that ferric and chloride ions cause significant acceleration in pitting. The bath was stagnant and the time of immersion of the sheets in the bath was long. The material failed under the combination of these adverse conditions. The paper brings out the details of the failure analysis carried out.     

High temperature alloy chloridation at 850°C

The resistance of eight alloys against chloridation was tested at 850°C in Ar/Cl₂ (2.5% Cl₂) for 15 min. Pre‐oxidation treatments were performed for 1 h and 8 h at 850°C in order to produce a thin, adherent and protective oxide scale able to improve the chloridation behaviour of the tested materials. The chloridised sample morphologies were compared to the morphologies observed on the non pre‐oxidised samples. The alloys containing a large amount of iron did not exhibit any chloridation resistance, even after pre‐oxidation, and were severely damaged. The nickel based alloys gave interesting results but were also attacked by chloride, probably by the “active oxidation” mechanism. The duration of the pre‐oxidation treatment plays an important role, since the 8 h pre‐oxidation appears more beneficial than the 1 h pre‐oxidation, to delay the chloridation, probably because of the best quality of the oxide layer grown during 8 h. For the nickel based materials, the effects of chloride appear less severe than for the iron‐based alloys, but are not stopped. The “active oxidation” mechanism is proposed to be responsible for the degradation of the tested materials.     

Dissolution corrosion of 4H‐SiC in lead‐bismuth eutectic at 550°C

The corrosion behavior of 4H‐SiC in lead‐bismuth (Pb‐Bi) eutectic (LBE) at 550°C is investigated. To clarify the effect of irradiation damage on corrosion, samples with and without Si⁵⁺ ion irradiation are contrastively evaluated. The main results show that dissolution corrosion occurs in both unirradiated and irradiated samples, while the irradiation damage can accelerate the corrosion rate. The corroded surface is characterized by the loss of C element and the formation of amorphous layers with a slight enrichment of Si atoms. The possible reasons are discussed.     

Effects of scratching on corrosion and stress corrosion cracking of Alloy 690TT at 58 °C and 330 °C

Scratched Alloy 690TT were characterized using X-ray stress analysis technique, nano-indentation, SEM, electrochemical and immersion tests. The stresses adjacent to the scratches, which resulted from scratching, were tensile parallel to the scratches and compressive perpendicular to the scratches. In the acidic sodium chloride solution at 58 °C, scratching caused E_corr to shift negatively owing to a transient anodic current accompanied by a larger difference between E_corr and the potential of zero charge. At 330 °C in the lead-containing caustic solutions, SCC initiated at the scratch-banks and scratch-beds after 30 days’ immersion. Scratched Alloy 690TT exhibited both IGSCC and TGSCC in initiation.     

The influence of iron and cobalt on the type II hot corrosion behavior of NiCr model alloys

The type II hot corrosion behavior of the alloys NiCr20, NiCr20Co10, and NiCr20Fe10 is investigated at 700°C in synthetic air + 0.5% SO₂ for up to 300 hr. Pure Na₂SO₄ and a eutectic mixture of MgSO₄–Na₂SO₄ are applied as deposits. The kinetics are investigated via dimensional metrology and correlated to the microstructural progression of the corrosion by examining the cross-sections. All alloys exhibit two-stage corrosion kinetics, with initially low and subsequently increased metal losses. Independent of the deposit composition, the metal loss after the longest exposure time is increased by the alloying element cobalt, whereas it is decreased for the iron-containing alloy. All alloys show increased metal losses when exposed to the MgSO₄–Na₂SO₄ deposit. The time to the propagation stage is similar for all tests. During the stage of low metal loss, all alloys develop a chromia scale and internal chromium sulfides. When the propagation stage is reached, chromium and nickel can be found along with oxygen and sulfur within the pit. Nickel is dissolved into the deposit, where it precipitates.     

High-temperature corrosion behavior of sputtered K38 nanocrystalline coatings with and without yttrium addition in molten sulfate at 900 °C

The high temperature corrosion behavior of sputtered Ni-based superalloy K38 nanocrystalline coatings with and without yttrium addition in molten sulfate (75 wt.% Na₂SO₄ + 25 wt.% K₂SO₄) was investigated at 900 °C in air. The results indicated that nanocrystallization significantly increased the corrosion resistance through the rapid formation of a protective oxide scale. The addition of yttrium in the nanocrystalline coating furthermore improved the corrosion resistance of the coating.     

Failure mechanism of MCrAlY coating at the coating‐substrate interface under type I hot corrosion

MCrAlY coatings are widely used to provide protection of hot component in modern gas turbine engines against high‐temperature oxidation and hot corrosion. Coating‐substrate interface, where the substrate is only partially covered by the ?coatings, is vulnerable to the hot corrosion attack. The accelerated degradation at the coating‐substrate interface can cause fast spallation of the coating, leading to the early failure of the gas turbine components. In this paper, MCrAlY powder was deposited on IN792 disks by high‐velocity oxygen‐fuel spraying. The hot corrosion behavior of the coated sample was investigated using (0.8Na, 0.2K)₂SO ₄ salt deposition at 900°C in lab air. Results showed a minor attack in the coating center, however, an accelerated corrosion attack at the coating‐substrate interface. The fast growth of corrosion products from substrate caused large local volume expansions at the coating‐substrate interface, resulting in an early coating spallation.     

显微组织对TiAl合金热腐蚀行为的影响

研究了均匀化和铸态TiAl合金在(Na,K)2SO4混合熔盐中的热腐蚀行为,讨论了α2相在均匀化和铸态TiAl合金热腐蚀中的作用机理.结果表明:含少量α2相的均匀化TiAl合金呈现良好的抗热腐蚀行为,腐蚀产物中的富Al2O3相成层状分布,在腐蚀产物/基体界面形成连续的富Ti硫化物; 而含有大量层状组织的铸态TiAl合金则遭受严重的热腐蚀,在腐蚀初期,α2 γ构成的层状组织优先腐蚀,铸态TiAl合金的腐蚀层主要由富Ti的氧化物和硫化物组成,形成的混合膜黏附性差,较易剥落.     

Corrosion behavior of Fe‐Si metallic coatings added with NiCrAlY in an environment of fuel oil ashes at 700°C

Electrochemical potentiodynamic polarization curves and immersion tests for 300 h at 700°C in a furnace have been used to evaluate the corrosion resistance of Fe‐Si metallic coatings added with up to 50 wt.% of NiCrAIY. The corrosive environment was fuel oil ashes from a steam generator. The composition of fuel oil ashes includes high content of vanadium, sodium and sulfur. The results obtained show that only the addition of 20 wt.% NiCrAlY to the Fe‐Si coating improves its corrosion resistance. The behavior of all tested coatings is explained by the results obtained from the analysis of every coating using electron microscopy and energy dispersive X‐ray analysis.     

The oxidation and hot corrosion behavior of tungsten-fiber reinforced composites

The oxidation and hot corrosion behavior of a tungsten-fiber, reinforced Ni~ 20Cr alloy has been examined under the following exposure conditions: (a) pure oxygen at 1 atm pressure; (b) sulfidation in H₂–10 %H₂S; (c) presulfidation in H₂–10 %H₂S followed by oxidation in oxygen; and (d) oxidation in 1 atm oxygen after precoating with approximately 1 mg/cm² of Na₂SO₄. Rapid oxidation of the tungsten fibers causes considerable distortion of the matrix and catastrophic degradation of the matrix follows. Inter diffusion between the matrix and the fibers is also important. During sulfidation, only the matrix forms sulfides, the fibers remaining unaffected. Consequently, presulfidation, although having a dramatic effect on the oxidation of the matrix does not have a damaging effect on the fibres. Equally, the presence of sodium sulfate is not critical, although severe oxidation of the exposed tungsten fibers is still observed.     

The corrosion of iron in pure SO2 under different pressures at 700°C

The reaction between iron and pure SO₂ under different pressures at 700 °C has been studied. The experiments carried out under static conditions showed a large scatter of kinetic data while the runs in flowing SO₂ at atmospheric pressure produced fairly reproducible results under convenient conditions. They differ, however, in some aspects from previous results for the same system; possible reasons for this effect are discussed. Finally, the formation of iron sulfide beyond the conditions of thermodynamic stability is examined by considering the possibility of the direct reaction of the metal with SO₂ rather than with the dissociation products of the latter.     

K4104镍基高温合金Al-Si渗层组织与抗热腐蚀性能

采用无机盐料浆法对K4104镍基合金进行Al-Si共渗,对Al-Si渗层的微观组织、显微硬度以及1 000℃抗氧化性能和900℃抗热腐蚀性能进行了研究.结果表明,Al-Si渗层连续,硬度比基体高;抗氧化和抗热腐蚀性能高于Al-Si共渗前.